The present invention relates to an electrode substrate for a fuel cell, and more specifically relates to an electrode substrate in contact with flow channels for a reactant gas, and wherein at least a part of the electrode substrate comprises a flexible carbon material as a reactant gas diffusion part. The term, "electrode substrate for a fuel cell" as used in connection with the present invention means all substrates which become an electrode for a fuel cell either by applying a catalyst to the substrate itself or by stacking on the substrate a porous electrode carrying a previously applied catalyst.
The flexible carbon material according to the present invention is obtained by carbonizing a composite material comprised of carbon fibers and a binding agent, wherein carbon lumps are derived from the binding agent and are dispersed in the matrix of the carbon fibers so as to restrain a plurality of the carbon fibers while yet slidably holding the fibers one to another by means of the carbon lumps.
In recent years, carbon materials made of carbon fibers as the basic material have been used in various industrial fields. Increasing usage of carbon fiber-based materials has, in turn, increased market demands for production of, and physical product improvements for, such materials. Carbon fiber-based materials are generally recognized as exhibiting excellent physical properties, for instance, heat-resistance, corrosion-resistance, conductivity, mechanical strength, and the like.
On the other hand, there have also been high demands for fuel cells for generating clean energy which can freely make and break electrical circuits, normalize the operation of thermal power generation or water power generation, and/or improve efficiencies of systems employing fuel cells.
Previously, a bipolar separator-type fuel cell has been provided with a bipolar separator obtained by mechanically ribbing an impermeable thin plate of graphite.
In addition, to the above-mentioned bipolar separator-type fuel cell, monopolar-type electrode substrates (i.e., a substrate in which one of the sides thereof is ribbed and the other side of which has a flat electrode surface so that a reactant gas diffuses from the ribbed side to the flat side of the electrode) are known.
Monopolar-type electrode substrates for a fuel cell have been proposed to be fabricated by press-molding short carbon fibers as the base (refer to U.S. Pat. No. 4,506,028). The electrode substrate obtained by this conventional method of production consists of one layer which has a uniform structure as a whole.
In an electrode substrate having a uniform single layer construction, (i.e., where the bulk density of the electrode substrate is large), since the gas-diffusion coefficient is small, rapid decrease of electrode substrate performance occurs because the limiting current density becomes smaller and the retained amount of electrolytic solution is insufficient. That is, such an electrode substrate exhibits a short life. On the other hand, in the case where the bulk density of the electrode substrate is small, the electrode substrate has insufficient mechanical strength, such as bending strength.
The present inventors have offered a composite electrode substrate which has been produced by press-molding and heat treatment (rather than by more difficult mechanical processing) using short carbon fibers as the basic material and providing the flow channels of a reactant gas near the center of the thickness of a porous carbonaceous gas-diffusion layer. The obtained composite electrode substrate exhibits excellent physical properties similar to those separators having a unitary body with a carbonized electrode substrate (refer to U.S. Pat. No. 4,522,895). According to the invention, it has become possible to use an electrode substrate which has a gas-diffusion portion exhibiting a large gas-diffusion coefficient (namely, a small bulk density). Furthermore, the contact resistance of the electrode substrate is reduced by a large margin as compared to conventional monopolar-type and bipolar-type substrates by uniting the separator in the body with the carbonized electrode substrate.
The electrode substrate of this invention obviates conventional ribbing and boring steps by using a binding material comprising a thermosetting resin of a specified carbonizing yield and a pore-regulator which is thermally decomposed at a temperature higher than the molding temperature. The porous carbonaceous layer is thus formed so that desirable continuous pores are formed in the porous carbonaceous layer but, as will be described later, it was impossible to avoid exfoliation of the porous carbonaceous layer from the gas-impermeable layer (the compact carbonaceous layer) in the steps of carbonization and calcination in the process of producing the electrode substrate. Particularly, when a larger substrate having a broad surface was produced, exfoliation occurred in spite of elevating the temperature to the calcining temperature, resulting in low production yields. Accordingly, an improvement of the process for producing the electrode substrate was definitely needed.
It was considered by the present inventors that exfoliation occurred in the calcination step (up to the maximum temperature of 3000.degree. C.) of the molded substrate due to the thermal expansion rate difference between the porous carbonaceous layer and the gas-impermeable layer when the substrate was subjected to elevated temperatures or to the thermal shrinkage difference between both layers when the calcined substrate was cooled to room temperature. Accordingly, methods of reducing or removing the expansion and shrinkage differences between the two layers were examined using a buffer layer interposed between the two layers, the buffer layer thereby compensating for the above-mentioned expansion and shrinkage differences.
As a result, the present inventors have examined a flexible graphite sheet which has relatively large expansion and shrinkage rates, improved adhesion properties, and is not highly gas permeable. By interposing the flexible graphite sheet between the porous carbonaceous layer of the above-mentioned electrode substrate and the separator and by joining the sheet to the two materials via a carbonizable adhesive, the present inventors have found that it is possible to prevent interlayer exfoliation which has hitherto been a problem and to produce a large-sized composite electrode substrate.
The flexible graphite sheet is obtained by subjecting naturally occurring graphite to acid treatment and further to heating, thereby expanding the interlayer of carbon-to-carbon bonding and compression-molding the thus form so-called expanded graphite particles. The thus obtained flexible graphite sheet can be made to be adhesive because of its scaly surface with some gas-permeability which allows impregnation of an adhesive and further, such a flexible graphite sheet is most suitable for absorbing expansion and shrinkage of the materials for the present invention due to the above-mentioned flexiblity properties.
As a result of further continued studies of the present inventors, it has been found surprisingly that the flexible carbon material (which will be defined below) is obtained by carbonizing a composite material comprising carbon fibers of not less than 1 mm in mean length which have been treated at a temperature of not lower than 1000.degree. C. and a binding agent.
In considering that development of carbon materials has focused upon the physical properties thereof, for example, mechanical strength, corrosion-resistance, conductivity, etc., it was not expected (nor was it intended) that the above-mentioned flexible carbon material could be obtained.
The present inventors have further found that in the cases where the above-mentioned flexible carbon material is used as the electrode substrate in the composite electrode substrate for a fuel cell, even in the case where the above-mentioned flexible graphite sheet is not used between the electrode substrate and the separator, the electrode substrate can be joined firmly with the separator without cracking, exfoliation, warping, etc. at the time the electrode substrate is produced.
The fundamental object of the present invention lies in the use of a novel flexible carbon material having a particularly novel microstructure as the electrode layer in the composite electrode substrate which is in contact with the flow channels for a reactant gas in a fuel cell. The flexible carbon material is obtained from a composite material comprising carbon fibers of not less than 1 mm in mean length which have been treated at a temperature of not lower tan 1000.degree. C. and a binding agent, wherein carbon lumps derived from the binding agent are dispersed in the matrix of the carbon fibers so as to restrain a plurality of the carbon fibers thereby slidably holding the fibers one to another.